This ingenious clock by Instructables member dodgey99 turns the knobs on an Etch A Sketch to write the time, then flips over to erase the markings and update.

First, he adapted a pre-existing body of code that used stepper motors to write random lines on an Etch A Sketch. He carefully rewrote this code so that it would not only compose specific lines, but reset the stylus on an analog device. Then he used pulleys to connect the knobs to a pair of overpowered stepper motors mounted a frame of acrylic sheets. An Arduino electronic controller directs the motors. Check out the impressive result in action: